Amp rating of Chokes/Inductors

[pjpoes]

Hey I was wondering how the power rating of chokes works. For instance a tube amp Choke rated for say 600 volts and 200ma's. Is that really based on a wattage rating, so at a lower voltage it would handle more current, or is the current rating fixed.

I want to build a pi filter for a Class AB solid state amplifier and the current demands are greater than 200ma, obviously. However the voltage is 63 volts, much lower, so I wondered if some of these standerd off the shelf units owuld work for my needs.

If not, I have been asking some places for quotes on custom made ones designed for my needs. Namely 100 volts, 5H, and 10 amps. Does that make sense, or am I specing them wrong?

[Boris_The_Blade]

For typical tube amps, chokes have high inductance ratings because of lower currents. If you really want a choke for a S.S. amp that needs, say 4 amps, you can have a much lower rated choke. Chances are there are not very many 5H 4 amp chokes readily available because the windings will be thicker due to increased current requirement and the core size will be huge.

Rule of thumb is for choke input power supplies: the lower current that you need, the higher inductance you will need...and vice versa. This is because to allow the same ripple voltage buffer (for lack of better words), the L and di/dt will change (go up, and go down, or oppositely) in the standard inductor equation V(t) = L*[di(t)/dt], where V(t) is voltage across the inductor and i(t) is current through the inductor.

You can also consider the inductor as an impedance in the frequency domain. With tube amps, a typical load of tubes might present a high voltage supply with say, 1kohm. Conversely, a S.S. amp might present it's supply with a typical load of 20 ohms. An inductor represents an impedance of j*w*L. Since w is constant at 2*pi*60, L determines impedance. Say that we want a filter impedance of %50 of the load value (500ohms for the tube amp and 10 ohms for the SS amp...remeber this is only AC impedance, power is only dissipated int the winding resistance (plus a little core loss)). This would mean an inductance of 1.3 Henries for the tube amp but only 27mHenries for the S.S. amp.

In other words, look for a much smaller inductor and they will likely have higher current ratings. Don't worry too much about the voltage ratings unless you are designing for a tube amp. Operating an inductor past its current rating will saturate the core and cause inductance to drop. It also risks overheating the windings and fire (if way past the current rating).

hope this helps,

[pjpoes]

Thanks that was very helpful. Now that I know this I believe I could get away with an off the shelf part from Hammond, or possibly a much cheaper custom made one.

Now, is there any issue with the current rating? In other words, should it be rated for the entire possible draw of the amp, or lower?

[radtech]

The current rating is going to depend on the wire gauge used in the choke, since a tube amp uses higher voltage at lower current they can use relatively thin wire, whereas for a high current SS amp you'd need a choke with much heavier wire.

The DC power dissipated in the choke will be [I squared * R] where R is the DC resistance of the choke and I is the DC current, so if you had a 50 ohm choke at 200mA it would only be 2 watts but at 2A it would be 200W!

[pjpoes]

well I understood the first part already, what I wondering was how to choose the current value. I was reading that in an LC where there is no C after the bridge, the Inductor must be able to handle the full current swings, but with a CLC that isn't the case, however this was discussing it around tube amps. I've seen all this discussed for solid state amps as well with regard to Class A amps such as Pass designs, but again he is using inductors rated far below the total draw of the amp. I believe in one of his projects they are 20mh at 3amps. The amps power supply however can produce over 10 amps.

[woody]

I don't know about a pi filter but for a simple choke input filter
the formulia L (henries) =V/I (miliamps) x.88 so for a 20v
supply pulling 4A we need a inductor of at least 5mh. Now if
you use an iron core inductor you need a rting greater than
4A because somewhere right above the rated current the thing
will start to saturate. In this case an air core inductor might be
a good choice but you need to watch the wattage disipated
in the choke.

[pjpoes]

If I want to figure out what I need for a supply that is 45v and 10 amps, I came up with one of at least 4mh and I was planning on using one that was rated for 10-25mh depending on what I can get.

Triad Magnetics makes some affordable ones, like 50 bucks each, so I could get two of them and be in good shape. Hmm would using two smaller ones in a CLCLC filter be better than one bigger one in a CLC filter?

I looked at an LC filter and it didn't seem to be as effective at reducing ripple as a CLC filter, but I was reading that they regulate voltage better? I would have thought that would go hand in hand, but I dont know, I'm just getting to understand this stuff.

[Tweeker]

You get better voltage regulation, much lower strain on the PT, and less diode noise.

[AndrewT]

Hi,
the output from a choke input filter is about Vdc=0.9Vac.
The output for a capacitor input filter is about Vdc=1.4Vac.
The inductor for a choke input filter should be designed to not overheat when on high demand and provide sufficient inductance to ensure correct regulation to achieve that 0.9Vac value.

There is a problem.
If the current demand falls below the lower limit for choke regulation, the voltage on the output starts to rise and will approach the 1.4Vac when the current demand is very low.

ClassAB amplifiers with a high ratio between quiescent current and Ipeak may fall into that category of PSU supply voltage swinging between 1.4 and 0.9 times Vac.
Choosing a high output bias design may avoid this problem.

Understand the design implications and take the necessary steps to ensure your supply voltage does not jump by that 50% unexpectedly.

BTW,
the reason you rarely find choke input filters in solid state amplifiers is that the choke will probably be as big as and as expensive as the main transformer and could be even bigger/more costly.

[pjpoes]

Could you further explain what you mean Andrew, what your saying would explain a problem I was having with my models. I'm basicly not sure how to "do the math" to ensure that doesn't happen.

When I modeled my power supplies on PSUD2 sometimes it would show me output that was more like 150 volts, other times it gave me the expected 60 volts. I thought it might be a glitch in the program, because if I shut it down and rerun it would give me what I expected.

I know the Inductors are typically large and pricey, but based on other information I heard, It seemed I could get away with a 5-10mh inductor that handles around 10 amps. Triad magnetics has some which are, I believe, designed for my purpose, and will cost around 55 dollars each or so. How do I figure out the minimum current that must be drawn in order for the inductors to maintain regulation.

Also, did I gather correctly that an LC will be better than a CLC? Thanks.